1. Field of the Invention
The present invention relates to apparatus and methods for heating steel workpieces, in general, and to apparatus and methods for rapidly heating thin continuously-cast hot workpieces to a temperature suitable for rolling, in particular.
2. Description of the Prior Art
In the past, roller-type or pusher-type hearths have been provided in furnaces to support numerous different forms of workpieces as, for example, a plurality of bars or slabs during heating within the furnace. Such workpieces are heated for a number of different reasons, all of which usually are characterized by the need to supply heated workpieces from the furnaces at a uniform temperature. The actual temperature to which these workpieces are heated depends upon the particular metalworking or treating operations but, generally, the workpieces are raised to a temperature above the critical temperature of the metal and frequently it is desired to raise the temperature of the workpiece to 2,000.degree. F. or higher.
As can be appreciated, the greater the thickness of the workpiece, generally the more treatment is required to reduce the workpiece to a final thickness--especially if the workpiece is to be formed into thin coiled steel strip. For obvious reasons, greater numbers of thickness rolling reductions of the workpiece add to the manufacture cost and time required for forming the final product.
In an attempt to reduce the number of steps required to produce finish strip from molten steel, the practice of continuously casting thin strip has been introduced. The thin casting of the workpiece into strip eliminates the ingot-to-slab formation step and the breakdown of the slab prior to formation of the finish strip as was required by previous conventional techniques. In the application of this technology, the relatively hot continuously-cast strip must be heat-soaked for sometime as it travels through a reheat furnace in order to further heat the strip to a temperature suitable for rolling. Similarly, cold strip workpieces must also be passed through such a reheat furnace before they too can achieve a temperature suitable for rolling.
A typical reheat furnace, in cooperation with the workpiece conveyance system associated therewith, is designed to sufficiently reheat workpieces of particular maximum thickness and/or minimum thermal conductivity. That is to say, a furnace capable of providing a given maximum heat output must be of a particular minimum length and/or the workpiece conveyance system associated therewith must be capable of being operated at a particular minimum speed in order to sufficiently heat-soak the workpieces.
For sufficient heat-soaking of conventional strip-like workpieces of up to approximately 2.5 inches in thickness, the typical reheat furnace may be designed to extend for lengths of 500 feet or greater, or the workpiece conveyance system must be capable of being slowed to relatively slow speeds to properly heat-soak the workpieces.
To more effectively utilize continuously-cast thin strip technology, an advantage exists for a furnace design and heating method which will more quickly raise the strip to a suitable rolling temperature without causing harm to the strip, thereby permitting: 1) a reduction of the required length of the furnace, and/or 2) an increase in the conveyance speed of the strip conveyance system associated with the furnace. Such reduction in the required length of the furnace will reduce construction, maintenance and energy consumption costs of the furnace while at the same time reducing the time in which the strip spends in the furnace. Moreover, increasing the conveyance speed of the associated conveyance structure further reduces the time which the strip spends in the furnace. In either event, a material decrease in the time which the workpieces spend in the furnace translates to a material reduction in the time required to form the molten steel into finished strip.
The design of the furnace will preferably be of a modular construction for relatively uncomplicated and economical application to both new furnace constructions as well as for retrofitting of existing furnace constructions.
It is therefore an object of the present invention to provide a furnace design and associated heating method for rapidly and effectively heating thin continuously cast hot metal workpieces to a suitable temperature for rolling without causing harm to the workpieces.
It is a further object of the present invention to provide a furnace design and associated heating method which will reduce the required length and, therefore, the construction, maintenance, and fuel consumption costs associated with steel workpiece reheat furnaces.
It is a further object of the present invention to provide a furnace design and associated heating method which reduces the time a workpiece is required to reside in the reheat furnace in order to obtain a temperature suitable for rolling, hence reducing the time in which the workpiece may be processed into a final product.
It is yet a further object of the present invention to provide a furnace design which is of a modular construction for relatively uncomplicated and economical application to both new furnace constructions as well as for retrofitting of existing furnace constructions.
Still further objects and advantages will become apparent in light of the attached drawing figures and written description of the invention presented hereinbelow.